docs: def branch_lengths(self, terminal=True, internal=True):
          '''Generator over the lengths of the selected branches of this ``Tree``. Edges with length ``None`` will be output as 0-length
  
          Args:
              ``terminal`` (``bool``): ``True`` to include terminal branches, otherwise ``False``
  
              ``internal`` (``bool``): ``True`` to include internal branches, otherwise ``False``
          '''
          if not isinstance(terminal, bool):
              raise TypeError("terminal must be a bool")
          if not isinstance(internal, bool):
              raise TypeError("internal must be a bool")
          for node in self.traverse_preorder():
              if (internal and not node.is_leaf()) or (terminal and node.is_leaf()):
                  if node.edge_length is None:
                      yield 0
                  else:
                      yield node.edge_length